Stannous chloride reduction–filtration for hexavalent and total chromium removal from groundwater

Abstract

Hexavalent chromium (Cr(VI)) reduction using stannous chloride (SnCl 2 ) has emerged as a possible alternative to chromium treatment technologies such as strong base anion exchange. In an effort to target not only Cr(VI) reduction but, ultimately, total chromium (Cr(T)) removal, SnCl 2 addition followed by rapid sand filtration was tested at the pilot scale on a groundwater with a naturally occurring Cr(VI) concentration of 0.090 mg/L. A SnCl 2 dose of 1.5 mg/L, followed by filtration, was able to consistently remove Cr(T) to less than 0.010 mg/L following an initial ripening period, with limited head loss for 10 sequential 17-to 25-hr filter runs. Total tin and turbidity removal were similar, decreasing to below 0.050 mg/L and raw water levels, respectively. Analysis of filter sand following backwashes and three different material pipe segments that were exposed to unfiltered water dosed with SnCl 2 indicated the accumulation of Cr and Sn on surfaces, which remains a concern.

Improvements in the reduction coagulation filtration process for hexavalent chromium treatment

“…Following the reduction step during Fe(II)-RCF, excess Fe(II) is purposefully oxidized to Fe(III), or ferric hydroxide (Fe(OH) 3 (s)), by dissolved oxygen (DO) via aeration or free chlorine (Cl 2 ) addition, after which particulate Fe(OH) 3 (s) and Cr(OH) 3 (s) can be removed by filtration. In addition to improving filtration, the oxidation step also ensures compliance with the EPA/DDW secondary MCL (SMCL) for total iron (Fe(T)) of 0.3 mg/ L. For Sn(II), filtration is also the final step but Cr(VI) reduction results in Sn(II) oxidation to stannic tin (Sn(IV)), not a known coagulant, prior to forming stannic oxide (SnO 2 (s)), in which case Sn(II)-RF is a more appropriate term for the process (Kennedy et al, 2020). Oxidation of excess Sn(II) to Sn(IV) by DO or free Cl 2 is not currently required as there is no EPA/DDW MCL or SMCL for total tin (Sn(T)).…”

“…Another potential opportunity for cost savings could result from the use of Sn(II), or more specifically stannous chloride (SnCl 2 ). Recent research by Kennedy et al (2018Kennedy et al ( , 2020, Henrie et al (2019), andNguyen et al (2020) for several Cr(VI) containing groundwaters ($10-100 μg/L) confirmed the reduction times are likely less than 1 min for reduction of Cr(VI), independent of the initial Cr(VI) concentration, to below the California rescinded MCL of 10 μg/L. Lower Sn(II):Cr(VI) mass dose ratios than Fe(II) were also realized, approximately 11:1-18:1, while maintaining the filterability of particulate SnO 2 (s) and Cr(OH) 3 (s), albeit following filter ripening (Kennedy et al, 2020).…”

“…Recent research by Kennedy et al (2018Kennedy et al ( , 2020, Henrie et al (2019), andNguyen et al (2020) for several Cr(VI) containing groundwaters ($10-100 μg/L) confirmed the reduction times are likely less than 1 min for reduction of Cr(VI), independent of the initial Cr(VI) concentration, to below the California rescinded MCL of 10 μg/L. Lower Sn(II):Cr(VI) mass dose ratios than Fe(II) were also realized, approximately 11:1-18:1, while maintaining the filterability of particulate SnO 2 (s) and Cr(OH) 3 (s), albeit following filter ripening (Kennedy et al, 2020). Therefore, with a fast reduction time and lack of coagulation/ flocculation time and free Cl 2 oxidation requirements compared to Fe(II)-RCF, Sn(II)-RF has the potential to reduce overall capital and operation and maintenance (O&M) costs, although SnCl 2 chemical costs would likely have to decrease substantially to give it the advantage over FeSO 4 or ferrous chloride (FeCl 2 ).…”

“…Lower mass doses compared to Fe(II)-RCF and no oxidation of excess Sn(II) to Sn(IV) likely led to overall less particle formation and slower headloss accumulation, leading to longer t R . For comparison, a UFRV value of approximately 130 m 3 /m 2 (3,100 gal/ft 2 ) (OWR value of 89%) was realized by Kennedy et al (2020), and t R could have been extended based on consistent Cr(T) removal. In the case of scenario F, it should be noted that prior to SnCl 2 testing the ceramic filter media had been tested with FeCl 2 and it is possible that incomplete removal of Fe(OH) 3 (s) during backwashing could have contributed to longer filter runs, improved particle removal, and Cr(III) adsorption during the SnCl 2 testing.…”

Influence of Particle Size and Zeta Potential in Treating Highly Coloured Old Landfill Leachate by Tin Tetrachloride and Rubber Seed

“…The removal achieved was relatively higher. In addition, another study conducted by Kennedy et al, (2020) [ 23 ] found that the use of SnCl 2 together with rapid sand filtration in a pilot scale in the treatment of groundwater achieved a consistent removal of chromium (Cr) from the groundwater. Moreover, Zepeda et al, (2018) [ 24 ] applied the usage of tin oxide (SnO 2 ) synthesised from SnCl 4 , finding that it allowed the significant removal of both nickel (Ni 2+ ) and copper (Cu 2+ ) ions from an aqueous solution.…”